The following descriptions and examples are not admitted to be prior art by virtue of their inclusion in this section. This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
The present disclosure relates generally to methods and systems for non-contact measurements of acoustic signals via a borehole wall. In particular, the present disclosure relates to non-contact measurements of acoustic signals during seismic activities in oilfields or gasfields.
Downhole acoustic investigation has been used in studying the structure of an underground formation on the basis of acoustic waves measured at various locations in a borehole, as well as studying seismic activities due to natural or controlled events. Seismic sensor (receiver) arrays are installed in a borehole in order to benefit from low acoustic noise. VSP (Vertical Seismic Profiling), Cross-well or Single-well techniques may be employed in the art of seismic surveys.
These techniques use arrays of sensors arranged within the borehole. The latest technologies comprise shuttles equipped with geophones or accelerometers coupled to the formation and in some cases decoupled from a tool body in order to provide a good quality measurement. Although decoupling the sensor from the tool body may prevent the propagation of undesirable noise, coupling of the sensor to the formation is often hindered by the size of the sensor packaging and the compliance of the formation itself. These limitations prevent the acquiring of signals exempt from spurious modes and thus may limit the frequency bandwidth of the acquired signal.
Many of the above-described techniques require the coupling of the sensor body to the formation and in some cases, complex tool architecture is designed in order to achieve this condition. As a result, the deployment of such tools may require to anchor the tool against the formation for each depth investigated. While the tool is stationary the signal is acquired. For the next measurement depth, the anchoring is released, the tool moves up to the next stage and is re-anchored so as to be in contact within the formation, and so on.